1. Field of the Invention
The present invention relates to a method for writing data into an image repetition memory of a data display terminal under the control of a memory access control, the data display terminal exhibiting an electron beam tube and an image control unit having at least two line buffers designed as clock interfaces.
2. Description of the Prior Art
Given a data display terminal which comprises an electron beam tube, refreshing the representation with, for example, approximately 50 Hz is required for a flicker-free presentation. The data to be displayed must be deposited in an intermediate memory, from which the data must always be called in the rhythm of the image repetition. Such an intermediate memory is called an image repetition memory.
The image repetition memory must, on the one hand, be available to the display terminal for the regeneration operation, but the image repetition memory must also be able to accept external data, for example from a data memory, when the image on the picture screen is to be altered. If the regeneration operation is not to be interrupted given a change of picture, then the inscription of new data must be chronologically interleaved with the read-out of the data for the image regeneration.
Such an interleaving can occur, for example, in such a manner that the image repetition memory is read during a forward beam scan, whereas the overwriting of the image repetition memory occurs during the beam flyback. As is known, the beam flyback time is shorter than the forward beam scanning time. Therefore, the sum of all forward beam scan times during an image cycle is significantly smaller than the sum of all beam flyback times.
Particularly given display terminals with a high character and line plurality or given display terminals in which additional control data are read into the image repetition memory with each datum to be displayed, the available beam flyback times during an image cycle can be insufficient for overwriting a memory area or for overwriting the overall memory area. Therefore, the overwriting must occur in a plurality of image cycles. For this purpose, a new image can only be slowly constructed. In addition to the loss of time, the slow image construction can have a disruptive effect on an operator.
A data display terminal is known from the German published application 30 26 225, fully incorporated herein by this reference, in which the characters to be displayed are the image repetition memory. The characters are supplied by way of two buffers of an image control unit, the buffers being designed as clock interface devices. Thereby, respectively one of the buffers can be connected to the background memory and the other can be connected to the image control unit. The image control unit controls the two buffers in such a manner that, simultaneously with the display content of the one buffer on the picture screen, the other buffer is loaded by the background memory with the characters to be subsequently displayed.